Universe's first stars may have been twins

A good fraction of the universe's first stars may have been born in pairs, a new study suggests.

Since each star in a pair is likely to be smaller than a single star created from the same natal material, the work may help explain why so far no evidence has been found for exotic physical processes thought to occur in super-heavy stars from the early universe.

It may also mean the star pairs could be detected by the gravitational waves they would emit at the end of their lives.

Not much is known about the universe's first stars. It is theoretically possible to see them with telescopes by looking for objects that are extremely far away, since their light takes billions of years to reach Earth. But today's telescopes are not powerful enough to see the dim objects.

Computer simulations suggest the ancient beacons, called population III stars, were extraordinarily massive, ranging from 30 to 300 times the mass of the sun. By contrast, the average Milky Way star is just 0.8 times the sun's mass.

Natal clouds

But until now, the simulations could not produce a phenomenon that is commonly seen elsewhere – stellar pairs.

"Multiple star formation had been proposed as a mechanism for forming the first stars in the universe, but this is the first time we've seen it in cosmologically realistic conditions," says Matthew Turk of the Kavli Institute for Particle Astrophysics and Cosmology in Menlo Park, California.

Turk and colleagues simulated the conditions 20 million years after the big bang, when the universe was a soup of mostly hydrogen and helium gas. They ran five simulations – each with slightly different initial distributions of gas.

The simulations modelled what would happen over the first 190 million years after the big bang, enough time for gravity and cooling gas to condense down into cores of dense gas that could eventually evolve to form stars.

Two from one

In one simulation, the team was surprised to see that a cloud had fragmented to form two such cores. Like pizza dough, the spin of the cloud seemed to contribute to tearing it apart. Hydrogen molecules created as the cloud collapsed gravitationally also helped cool the gas, enabling it to clump up even more easily.

Many more simulations will be needed to reveal just how common population III star pairs might be. The fraction of the stars that are born in pairs is "probably less than half but probably more than 5 per cent," Turk says.

If a significant fraction of population III stars are born in pairs, it could resolve a strange discrepancy between simulations and observations of the stars' descendents, which can still be found roaming the Milky Way.

Previous simulations suggested that a large fraction of population III stars may be so massive they are completely torn apart in exotic explosions called pair-instability supernovae.

Telltale pattern

In stars 140 to 260 times the mass of the sun, the high temperatures and pressure conditions are ideal for converting light into pairs of particles, each comprising an electron and its anti-matter counterpart, a positron.

This causes a drop in pressure that makes the star unstable and causes it to contract under its own weight. Eventually the process ignites runaway nuclear reactions that rip the star apart.

These reactions also produce elements with a particular pattern of abundances – elements like nickel with an even number of protons are far more common than those with an odd number of protons.

Strangely, this pattern has not been seen in the oldest stars in the Milky Way, which descended from population III stars and would have incorporated their remains.

Two smaller stars

Since each of the two stars born from a cloud would likely be less massive than a single star, neither star in the pair might be massive enough to trigger the exotic explosions and the patterns of chemical abundances.

Finding that population III stars can be born in pairs "nicely makes this discrepancy go away, which is really exciting", says co-author Brian O'Shea of Michigan State University.

"But there are other ways to get around that problem," says Volker Bromm of the University of Texas in Austin, who was not a part of the team.

He says the reason such chemical patterns have not been found is because existing surveys of Milky Way stars are not looking at the stars that would have them. Existing surveys hunt for stars that are very low in heavy elements, but he says evidence for extremely heavyweight stars would be found in stars that have higher abundances of such elements.

Slingshot away

If the first stars did form in pairs that orbited each other, over time they would be expected to spiral towards each other as they radiated energy away by emitting gravitational waves, or ripples in space-time. Eventually, they would merge, producing a burst of gravitational waves that could one day be detectable from Earth.

Bromm adds that stellar pairs might make it difficult for supermassive black holes – which are thought to exist at the heart of large galaxies – to form by mergers between smaller black holes. That's because each star in the pair could become a black hole when it died, and a tightly orbiting pair of black holes could slingshot other black holes away instead of absorbing them, he says.

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Primordial clouds of gas can give birth to single stars and also to twins, a new simulation suggests (Illustration: Science/AAAS)